The Grundfos is a circulator pump, and the pipe is the recirculation line whic is connected to the far end of the hot water system. The circulator move the hot water through the system and allows the faucets to have hot water almost instantly when they are operated. The screw, if it is on the back side is the access to the motor shaft.

: I am having trouble identifying a particular part in line with my hot water heater. I have a conventional gas-fired hot water heater with the cold inlet on top and the hot outlet on top. The drain is located along the side of the heater near the bottom. A pipe is attached to the drain which leads to a tee. On one side of the tee is a valve/faucet which is the actually drain. On the other side of the tee, however, is another pipe which runs into trhe house, although I do not know to where. In line on this same pipe is a device (maker is Grundhof although I am not sure of the spelling) which is plugged into a 120v outlet. On the side of the device is a screw for adjusting what appears to be impedence, although I am not sure. What is this device? A pump? Why is there a line going back into the house from the hot water heater drain? Thanks.